A centrifugal compressor configured to compress a gas is widely known as a rotary machine. In such a centrifugal compressor, an impeller is installed in a casing, and a gas suctioned from a suction port is compressed by rotation of the impeller and ejected from an ejection port.
Here, an impeller eye seal is installed in a impeller eye, an intermediate stage seal is installed between stages of the impellers, and a balance piston section seal is installed at the last stage to reduce a leakage amount of the gas compressed by the impeller. Then, various seals such as a damper seal, a labyrinth seal, and so on, may be used as the seal.
The labyrinth seal has a plurality of protrusions protruding from an annular standstill side member opposite to a rotating rotary shaft with a gap interposed therebetween toward the rotary shaft. In the labyrinth seal, leakage of a fluid can be reduced by generating a pressure loss in the fluid flowing through the vicinity of a tip of the protrusion. A honeycomb seal, a hole pattern seal, or the like, is known as the damper seal. For example, in the hole pattern seal, in the annular standstill side member having the gap with the rotary shaft, a plurality of hole sections are formed at an opposite surface thereof opposite to the rotary shaft. The leakage of the fluid is reduced by the pressure loss generated in the hole section.
The hole pattern seal has a larger damping effect of vibrations than the labyrinth seal, and is excellent in stabilization of vibrations of the rotary shaft. Meanwhile, the labyrinth seal can further reduce the leakage amount of the fluid in comparison with the damper seal.
A seal structure using the hole pattern seal is disclosed in Patent Literature 1. In the seal structure of Patent Literature 1, as both of the labyrinth seal and the hole pattern seal are used, various advantages are utilized to improve rotation performance of the centrifugal compressor while improving seal performance.
The rotary shaft of the rotary machine is supported by a bearing. Then, when a destabilization force generated from the seal or the impeller with respect to the damping force obtained by the bearing is increased, unstable vibrations occur with a natural frequency of the rotary machine determined by a load or a revolution number. As a result, the rotary shaft is vibrated. In consideration of the above-mentioned problems, anisotropy of the stiffness (anisotropic stiffness) of the bearing is provided by the stiffness of the bearing being varied in the circumferential direction. Then, a method of intentionally vibrating the rotary shaft in an oval shape and suppressing unstable vibrations of the rotary shaft is employed.